1. Field of the Invention
The present invention relates to a route searching apparatus for searching for a route to a destination from the current position of a vehicle. Particularly, it relates to a route searching apparatus that facilitates a process of performing route searching.
2. Description of Related Art
Conventionally, in a navigation system that detects the current position of a vehicle and displays a mark indicating the current position on a roadmap displayed on a display screen, a route searching apparatus for searching for an appropriate route from the current position of the vehicle to a destination is used. This route searching apparatus calculates a cost required for each road link to be searched for that exists between the current position of the vehicle and the destination by using road link information provided to each road link between nodes. When the route searching apparatus then finishes calculating the cost required for each of all road links to be searched for, the route searching apparatus selects road links from among all the road links so that the total cost is minimized and connects them with one another so as to determine the appropriate route to the destination.
In this case, because the route searching apparatus acquires, as the appropriate route, a series of the selected road links that are connected with one another, the series of the connected road links extending from the current position to the destination, the route searching apparatus cannot adopt, as part of the route, road links corresponding to restricted roads where vehicles are not allowed to enter, such as road links corresponding to one-way roads or pedestrian roads. Therefore, when determining that all the road links to be searched for include one or more restricted road links based on information about connection between road links, the route searching apparatus performs route searching while excluding the one or more restricted road links.
By the way, such restricted roads where vehicles are not allowed to enter include not only roads where restrictions are placed on all vehicles but also roads where restrictions are limitedly placed on specific vehicles according to conditions of the vehicles. For example, car pool lanes employed by a road system mainly seen in a large city of North America are roads where restrictions are limitedly placed on specific vehicles according to conditions of the vehicles. Car pool lanes are lanes where only vehicles in which two or more occupants are riding together are allowed to travel, for example. A lane additionally disposed in a highway, a lane that is a shortcut disposed in an interchange, and so on are known as car pool lanes. A road system that employs car pool lanes gives people preferential treatment of making it possible for people to arrive at the destination in a short time if traveling in a car pool lane so as to encourage people to ride together in one vehicle and to decrease the whole traffic to reduce traffic congestion.
Most of prior art route searching apparatus don't use car pool lane information when performing route searching because the condition of a vehicle change according to the number of occupants riding together in the vehicle. In this case, prior art route searching apparatus assume that car pool lanes are restricted lanes where all vehicles are not allowed to travel, or intentionally set the cost of car pool lanes, which is used for the route determination, to very high so as to substantially exclude car pool lanes from targets of the route determination. Therefore, even if a searched-for route including one or more car pool lanes is a shortcut, the route is not acquired as the searching result.
As previously explained, prior art route searching takes only information about roads (i.e., road link information, connection information about connection between road links, etc.) into consideration, but doesn't take information about vehicles that travel on roads into consideration. In recent years, route searching apparatus for performing route searching in consideration of information about vehicles that travel on roads in order to support road systems provided with car pool lanes have been developed.
For example, Japanese patent application publication (TOKKAI) No. 2000-131085 discloses a route setting device and a navigation apparatus that can implement more appropriate route setting in order to support cases where whether or not a vehicle is allowed to travel is determined according to the circumstances of the vehicle. The route setting device and the navigation apparatus set a route to the destination in consideration of vehicle-associated information. For example, while the route setting device and the navigation apparatus assume that when one occupant or less is riding in the vehicle, the vehicle is not allowed to travel in car pool lanes, and then excludes car pool lanes from targets of the route determination, the route setting device and the navigation apparatus deal with car pool lanes in the same way as dealing with other roads and assume car pool lanes to be targets of the route determination when two or more occupants are riding together in the vehicle. Therefore, when two or more occupants are riding together in the vehicle, there is a possibility that an advantageous route to the destination is set as compared with the case where one occupant or less is riding in the vehicle. In other words, because car pool lanes often provide an advantage, such as a shorter approach to an exit of a highway or the like or a shorter distance over which the vehicle will travel in a highway or the like, for people riding together, unlike general lanes, it can be expected that a more appropriate route to the destination including one or more car pool lanes is provided.
Japanese patent application publication (TOKKAI) No. 2001-183159 discloses a navigation apparatus that enables a target vehicle to effectively travel in a specific lane which vehicles are limitedly allowed to enter or leave. The navigation apparatus reads map data including road information about enter/exit points of highways provided with car pool lanes from a DVD, and then stores the map data in a data buffer. The navigation apparatus has a route searching process unit that performs a route searching process by using the map data stored in the data buffer in consideration of whether or not the target vehicle is allowed to travel car pool lanes. When performing a route guidance using car pool lanes, a car pool lane guidance unit of the navigation apparatus provides guidance for instructing a lane change by using an image and a voice at timing when the vehicle reaches at a predetermined distance or less from an enter/exit point where the vehicle should change to another lane.
In the prior art technology disclosed in above-mentioned Japanese patent application publication (TOKKAI) No. 2000-131085 and Japanese patent application publication (TOKKAI) No. 2001-183159, the user sets information about the vehicle to the road searching apparatus in advance so that the road searching apparatus can perform route searching in consideration of the set information about the vehicle. Therefore, the user has an inconvenience of having to set a condition of the vehicle in advance regardless of knowing if such a road as a car pool lane on which determination of whether or not vehicles are allowed to travel can be performed according to the condition of the vehicle is included in the searched-for route from the current position to the destination.
Furthermore, in such a system in which whether or not vehicles are allowed to travel is determined according to the number of occupants riding together as a road system provided with car pool lanes, because the condition of a vehicle changes every time when the vehicle is driven and every time when one or more occupants get on or off, the user has an inconvenience of having to reset the condition of the vehicle on an as-needed basis.
In addition, because the user needs to set various conditions associated with the vehicle, a setting screen that allows the user to set various conditions to the route searching apparatus contains a lot of screens, and therefore the user needs to perform operations until a desired screen is provided. The route searching apparatus can be so constructed as to automatically detect and set a condition of the vehicle, such as the number of occupants riding together in the vehicle. A problem is however that the route searching apparatus needs a sensor or the like for automatically detecting the number of occupants riding together in the vehicle, and therefore increases in cost.